Streptococcus sanguis is the dominant microbial species in dental plaque of humans. At least two characteristics are believed to be instrumental in this ecological superiority - rapid absorption to enamel surfaces and inhibition of other oral bacteria. Because the expression of these characteristics varies considerably among the members of this species, only a fraction of the many strains of S. sanguis can be expected to play a significant role in dental plaque. In order to determine which strains are important to dentistry, a method must be made available for their identification. The principal objective of the proposed research is the development of a typing system with which new isolates can be identified as either unique strains or as duplicates of strains previously typed. This typing system will consist of two standard sets of streptococci. Members of one set serve as sensitive indicators for bacteriocin production. Operationally, a new isolate is incubated with each member of the sensitivity set and then scored for its ability to inhibit growth. An adequate number of indicators will be employed to assure that each individual strain will display a unique pattern of positive and negative responses. Similarly, the isolate will be typed by its sensitivity to the standard bacteriocin-producer strains.